The present invention relates to an electrical heating system, in particular for the heating of the interior of an instrument cabinet wherein heating system is protected against explosion.
Explosion-protected heating systems are known which take up little space. For example, in industrial plants often a number of measuring instruments connected via valve units to pipe lines are used. In the flow of fluid media, for example, there are pressure or flow measuring points, measured by installations of measuring transducers and valve blocks. Out of doors these installations are normally mounted in protective instrument cabinets and the inside of the instrument cabinet or the built-in instruments themselves are heated directly by thermostatically controlled heating systems. In general, such a heating system is expected to be highly efficient, to require little space, to be adaptable to different conditions, and to adjust precisely to a preset temperature while safety regulations, e.g. with respect to protection against contact and explosion protection. Safety regulations regarding contact safety and explosion safety must be followed in the use of the systems. A known heating system of this type (DE 38 14 145 C2) comprises a heating radiator with a receiving opening, a heating element with an excess-temperature fuse with a different triggering temperature connected in series. The heating element is placed in the receiving opening and is encapsulated. The heating element and the excess-temperature fuse are connected in series, and are connected to a temperature control system. A environmental temperature sensor is also connected to the control system as an indicator of actual temperature. Heating systems of this type must be adapted to actual application conditions, and must be sized so that limit temperatures are not exceeded when in operation with temperature control. However, under adverse conditions, and with the unfavorable marginal conditions, especially if the temperature control overshoots, inadmissible overheating of the heating system is possible in the absence of additional protections. In order to ensure explosion protection, the excess-temperature fuses interrupt the electrical heating circuit if limit temperatures are exceeded. Fuse switching elements that can be reset when their protection function has been triggered have been used in the past as excess-temperature fuses. Manually replaceable melting fuses are also known. When an excess-temperature fuse has been triggered its function could easily and rapidly be restored in the past by resetting the fuse or by replacing it.
According to new guidelines, and allowing some guideline transition time, only melting fuses are allowed for use as excess temperature fuses in cast-encapsulated heating system. It is no longer possible to reset or replace the fuse once it has been triggered if the fuse is encapsulated during casting, so that a cost-intensive replacement of such a heating system becomes necessary.
Accordingly, an object of the present invention is to provide such an electric heating system which is protected from an excess temperature and explosion in which complete replacement of the heating system can be avoided.
The above objectives are accomplished according to the present invention by installing a fuse-temperature sensor near a protective melting fuse which is encapsulated together with the heating element and fuse in the receiving cavity. The fuse-temperature sensor is connected to the temperature control. With this fuse-temperature sensor the current temperature of the melting fuse can be compared with a changeover temperature set in the temperature control. The changeover temperature generally corresponds to the trigger temperature at which the fuse melts and is selected so that it is at a desired temperature level below the trigger temperature of the melting fuse. The temperature control is designed to operate in the usual manner with a first regulating algorithm in a temperature range below the set changeover temperature as sensed by the fuse-temperature sensor while taking into account the environmental temperature sensor as a current-value indicator. The temperature controls recognize when the changeover temperature is reached and/or exceeded as sensed by the fuse-temperature sensor. The temperature control switches to a second control algorithm to lower the heating performance of the heating element to protect the melting fuse when the changeover temperature is sensed. After a subsequent drop below the trigger temperature, possibly to an undershoot level provided for in the control algorithm, the temperature control switches once more back to the first control algorithm.
The changeover temperature must be selected so that under unfavorable conditions, e.g. in case the temperature control overshoots the temperature in an initial heating phase before the temperature has been adjusted, the fuse trigger temperature is not reached due to the lowering of the heating performance of the heating element. In this way the melting, and thereby destruction, of the encapsulated melting fuse is prevented even under unfavorable operating conditions. The operation of the heating system is maintained and, usually, somewhat longer heat-up times may occur. The operation of the melting fuse as an explosion protection fuse is not affected by the additional use of the fuse-temperature sensor and the modified controls, and is maintained thereby. Thus, it can be seen that sufficient heating operation is available with the arrangement according to the invention under usual conditions and that provisions of explosion protection regulations are observed. Since triggering and melt of excess temperature fuses is prevented, controls as were necessary to determine whether the excess temperature fuses have been triggered can be omitted, as well as the cost of replacement of complete heating systems.
In one aspect of the invention, the arrangement according to the invention can be installed advantageously in known radiators designed as an elongated body with an axial receiving cavity and radially projecting heating fins. The radiator is preferably made of metal to achieve good heat conductivity. It is known to make the radiator entirely as a cast piece or, as described in DE 38 14 145 C2, to make the fins separately and to attach them to a base body. Advantageously, a low-cost NTC resistance can be used as the fuse-temperature sensor. For normal applications, it suffices to set the changeover temperature permanently in the control circuit. Better adaptation to different situations is possible if the changeover temperature is adjusted on the temperature control. Even with an adjustable temperature, the safety function of the encapsulated melting fuse is not affected. For normal applications the control algorithms can be permanently programmed. Advantageously the control algorithm may be designed to be adjustable for the temperature range below the changeover temperature and/or the control algorithm for the range above the changeover temperature.